The present invention relates to fuses in integrated circuits and, in particular, to multiple electric fuses for redundancy.
Fuses are devices extensively used in integrated circuits to provide a way to program, repair, or modify the operation of, an integrated circuit after the circuit has been manufactured. Typical applications for semiconductor fuses include programmability of memory (PROM, EPROM) and disablement/enablement of certain circuitry for redundancy purposes (memories), and the like.
The two main types of fuses in common use by the semiconductor industry are electric fuses and optical fuses. Optical fuses are blown (or open-circuited) using radiation (such as laser) while electric fuses are blown by an electric current flowing through the electric fuse. In many applications, electric fuses are preferred over optical fuses due to the complexity and time needed to blow optical fuses using radiation.
One problem that exists with electric fuses is that sometimes after an electric fuse is blown, the fuses can reform upon cooling or sometime thereafter. While additional and complex testing may detect such a defect, it is generally desirable to blow the fuse(s) and perform no additional testing (in most cases, the testing has already been performed prior to the blowing of the fuses). In addition, even though duplication of the step of blowing the fuse may sometimes bring success in re-blowing a fuse that has reformed immediately, there is still a substantial possibility that the fuse may reform again after packaging of the die or during use in the field. If this occurs, the integrated circuit will be (or become) defective and cannot be repaired, thereby reducing the yield or affecting the IC during customer operation.
Accordingly, there exists a need to increase the yield of integrated circuits (and decrease the likelihood of failure in the field) that utilize electric fuses therein by reducing the likelihood that a reformed electric fuse (reformed after blowing) will cause a fatal defect.
According to the present invention, there is provided a fuse circuit in an integrated circuit. The fuse circuit includes a first fuse of conductive material and having a first side coupled to a first node and a second side coupled to a second node, and a first control device having a first terminal, a second terminal, and a control terminal, such that the first terminal is coupled to the first node, the second terminal is coupled to a first reference voltage, and the-control terminal is coupled to a first fuse control signal whereby the first control device is operable, in response to the first fuse control signal, to cause an electric current to flow through the first fuse sufficient to blow open the first fuse. The fuse circuit also includes a second fuse of conductive material and having a first side coupled to the second node and a second side coupled to a second reference voltage, and a second control device having a first terminal, a second terminal, and a control terminal, such that the first terminal is coupled to the second node, the second terminal is coupled to a third reference voltage and the control terminal is coupled to a second fuse control signal whereby the second control device is operable, in response to the second fuse control signal, to cause an electric current to flow through the second fuse sufficient to blow open the second fuse.
In another embodiment of the present invention, there is provided a circuit for providing fuse redundancy in an integrated circuit. The circuit includes a series-coupled plurality of fuses of conductive material with one end coupled to a first voltage reference. Each of the plurality of fuses is coupled to a fuse control device for controlling an electric current flowing through the fuse in response to a fuse control signal. A second end of the plurality of fuses is coupled to the first voltage reference when none of the plurality of fuses are blown and decoupled from the first voltage reference when at least one of the plurality of fuses is blown.
In yet another embodiment, there is provided a method of blowing electric fuses. The method includes the steps of generating a first electric current flowing through a first fuse and a second fuse, with the electric current having a magnitude sufficient to blow the first fuse. A second electric current is generated that flows through the second fuse sufficient to blow the second fuse, such that a first end of the second fuse is coupled to a first voltage reference.